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Think that size tank is still too small for sting rays...
They need a really really large tank, even the tea cups. If you see them in the LFS, they look really depressed--they can't move around.
I wouldn't put rays in anything under 300 gallons. Liveaquaria.com recommends 180 for one SW species, and 300 Gallons for the other. These can be larger than FW species, but the FW species still get quite large. Think about a rainbowfish tank, or a discus tank. Or think about a puffer tank (they're really fun!).
You will be bored silly. they mostly just sit there on the bottom and do nothing. I highly reccomend doing something else with that tank. A 90 is good sized, your options are many.
I agree with madasafish, that tank is to small for stingrays. Your sump will give you a great filter but i would still be worried that the rays would not have much room.